Phonolite

Phonolite is an uncommon shallow intrusive or extrusive rock, of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grained) to porphyritic (mixed fine- and coarse-grained). Phonolite is a variation of the igneous rock trachyte that contains nepheline or leucite rather than quartz. It has an unusually high (12% or more) Na2O + K2O content, defining its position in the TAS classification of igneous rocks. Its coarse grained (phaneritic) intrusive equivalent is nepheline syenite. Phonolite is typically fine grained and compact.

The name phonolite comes from the Ancient Greek meaning "sounding stone" due to the metallic sound it produces if an unfractured plate is hit; hence, the English name clinkstone is given as a synonym.

Formation

Unusually, phonolite forms from magma with a relatively low silica content, generated by low degrees of partial melting (less than 10%) of highly aluminous rocks of the lower crust such as tonalite, monzonite and metamorphic rocks. Melting of such rocks to a very low degree promotes the liberation of aluminium, potassium, sodium and calcium by melting of feldspar, with some involvement of mafic minerals. Because the rock is silica-undersaturated, it has no quartz or other silica crystals, and is dominated by low-silica feldspathoid minerals, such as nepheline, more than feldspar minerals. Phonolites typically form under specific conditions of low pressure and relatively high temperatures.